Why isn't my custom theme working?
Upgrade to Carbonize v1.1 to fix this issue.
Download and install Carbonize v1.1 from the Mac App Store to fix this issue and enable instant exports.
Carbonize relies on
WKWebView (which is basically Safari) to perform all the web related tasks for Carbonize. There’s a limitation on how much information it can send to the server that renders the stylized code images. Adding a background image currently crosses that limit.